This invention relates to engine control for a crankcase scavenged, two-stroke engine, and more particularly to a control system for reducing the exhaust gas hydrocarbons emitted from such an engine at and slightly above idle speed and low power requirements, by controlling the quantity of intake air and fuel delivered to the engine.
In conventional four-stroke engines, as operator demand for engine power is increased from idle, the standard practice is to increase the amount of air per cylinder supplied to the engine. This produces an increase in the delivered fuel per cylinder, maintaining the appropriate air-fuel ratio to achieve the desired engine performance and emission objectives.
The structure and operation of crankcase scavenged, two-stroke engines differ in many respects from that of conventional four-stroke engines. One of the major differences concerns the manner in which fresh air is inducted, and burned fuel is exhausted by the engines. Conventional four-stroke engines have intake and exhaust valves within the cylinders to accomplish these tasks. Crankcase scavenged, two-stroke engines, on the other hand, do not employ intake and exhaust valves. Instead, inlet and exhaust ports open directly into the walls of the engine cylinders. The inlet and exhaust ports are covered and uncovered by movement of the piston within the cylinder. As combustion is initiated, the piston moves in its downstroke within a cylinder, uncovering the exhaust port to release burned fuel, and then uncovering the inlet port to enable the entry of a fresh charge of air, which assists in driving out the burned fuel.
One of the major problems associated with crankcase scavenged, two-stroke engines has been the high level of hydrocarbons present in the engine exhaust gas. At speeds near engine idle, with light operator induced loading, the level of exhaust gas hydrocarbons is highly dependent upon the amount of air per cylinder delivered to the engine. This relationship is thought to result from the absence of valves in the two-stroke engine, and the near simultaneous opening of both inlet and exhaust ports in a cylinder wall for brief periods during the engine operating cycle. Presumably, an excessive amount of air flowing through the inlet port, drives fuel, which is not fully combusted, out the open exhaust port, thereby increasing the hydrocarbon content in the exhaust gas.
If the conventional practice is followed in controlling the near idle operation of a crankcase scavenged, two-stroke engine, by increasing the mass air per cylinder flowing to the engine, upon operator demand for output power, the level of hydrocarbons in the engine exhaust will be unreasonably high. Consequently, a need exists for an alternative engine control scheme for crankcase scavenged, two stroke engines operating at speeds near idle, with light operator induced loading.